A polyester whose main repeating unit is ethylene terephthalate (hereinafter sometimes abbreviated as PET) has been employed as a material of a container for a carbonated beverage, juice, mineral water and the like by virtue of its excellent characteristics such as transparency, mechanical strength, heat resistance and gas barrier properties, and the prevalence thereof is remarkable. In these applications, a polyester bottle is hot-filled with a beverage sterilized at a high temperature and moreover a polyester bottle is sterilized at a high temperature after being filled with a beverage. However, in an ordinary polyester bottle, a problem of contraction and deformation occurs at the time of such a hot filling treatment or the like.
As a method for enhancing the heat resistance of a polyester bottle, there has been proposed a method for subjecting the bottle cap part to a heat treatment to heighten the degree of crystallinity or subjecting the stretched bottle to heat fixation. In particular, in the case where the crystallization in the cap part is insufficient or there is a great variation in the degree of crystallinity, the sealing with a cap is unsatisfactory and leakage of the contents is sometimes caused.
In the case of a polyester bottle for beverages requiring hot filling such as a fruit juice beverage, oolong tea and mineral water, as described above, a method for subjecting a preform or the molded cap part of a bottle to a heat treatment to crystallize it is commonly employed. With regard to such a method, that is, a method for subjecting the cap part and the shoulder part to a heat treatment to enhance the heat resistance, since the productivity is largely affected by the time and temperature for performing the crystallization treatment, it is preferred that the raw material be PET having a high crystallization speed which can be treated at a low temperature and in a short time.
On the other hand, the barrel part is required to be transparent even after being subjected to a heat treatment on molding so as not to deteriorate the visibility of the color tone of the bottle contents, and the cap part and the barrel part are required to have characteristics contradictory to each other.
Moreover, in order to enhance the heat resistance of the bottle barrel part, a method for removing a molded item obtained by stretch blow molding from a blow mold for molding and then holding the molded item in a mold for heat fixation to be subjected to heat fixation (for example, see Patent Document 1), and a method for performing heat fixation simultaneously with stretch blow molding in a mold for blow molding (for example, see Patent Document 2) have been known. Moreover, a method for performing a heat treatment simultaneously with stretch blow molding in a primary mold, removing the molded item, and then subjecting the molded item to blow molding in a secondary treatment mold without subjecting the item to cooling (for example, see Patent Document 3) has also been known.
However, when the long-term continuous molding of bottles is continued by using only one mold in such a method, with the long-term operation, the resulting bottle is whitened, the transparency deteriorates, and only bottles having no commercial value are obtained. It has been found that this is because deposits resulting from PET stick to the mold surface, and as a result thereof, the deposits become mold stains, and these mold stains are transferred to the surface of a bottle. Particularly, in recent years, the molding speed of the bottle has been increased for reduction in costs, and shortening of the heating time for crystallization of the cap part and the mold stains have become a greater problem from an aspect of productivity.
Moreover, a stretched film is produced by extruding PET into a sheet-like object and stretching the object in the biaxial directions. At this time, there is a problem that deposits resulting from PET stick to a cooling roll and a stretching roll for the unstretched film, the runnability for production of a sheet-like object or a stretched film is adversely affected, and moreover, the transparency of the resultant product also deteriorates.
To cope with the problem of such mold stains, a method for reducing the cyclic trimer which is a main component of the deposits on the mold surface by previously subjecting PET to solid phase polymerization has hitherto been performed. In this method, since the cyclic trimer is regenerated at the time of remelting and parison molding, the effect is insufficient. Moreover, a method for treating a polyester with water of 90 to 110° C. to suppress the activity of the catalyst and controlling the generation of the cyclic trimer at the time of parison molding (for example, see Patent Documents 4 and 5) has been disclosed. However, in this method, the fixed cost is increased since the facility investment for the water treatment is required. Moreover, the mold stains are roughly reduced, but the reduction is still insufficient, and it has been found that there are cases where a sufficient effect cannot be obtained. Particularly, in the case of producing a small heat-resistant blow-molded article by the continuous long-term molding with a high-speed molding machine, the accumulation of deposits on the heated mold is quite significant. Since there occur problems such that great labor is required for cleaning the mold as well as the transparency of the resultant molded article considerably deteriorates due to the deposits, a solution by a less expensive method has been desired.
Furthermore, there has been proposed a polyester providing a blow-molded article which undergoes little thermal deterioration, but it has a problem that the transparency of the resultant product also deteriorates (for example, see Patent Document 6).
Moreover, a polyester sheet (hereinafter, sometimes referred to as “C-PET sheet”) made of a PET resin composition (hereinafter, sometimes referred to as “C-PET”) prepared by adding a small amount of a polyolefin resin to PET and a C-PET sheet made of a PET resin composition prepared by blending small amounts of a polyolefin resin and a heat stabilizer or a crystal nucleating agent into PET (for example, see Patent Documents 7 to 10) are excellent in moldability, impact resistance, gas barrier properties and heat resistance, and a container made of a C-PET sheet has been widely used as an inexpensive container for a pressurized heat-treatment.
The above-described C-PET sheet is continuously thermoformed by a heated mold in a vacuum molding machine or the like. At this time, PET oligomers, a polyolefin resin and oligomers thereof and the like gradually stick to and are accumulated on the mold, and as a result thereof, the mold releasability between the heated mold surface and the C-PET sheet deteriorates and the gloss on the inner surface of a container deteriorates, for which a solution has been desired.